WO 95/11100 A1 describes a laser sintering method as a special case of a layer-wise additive manufacturing method. In case a powder is used as starting material for the manufacture, which powder is sintered and melted, respectively, by the introduction of heat energy, the building process usually is carried out such that a CAD model of the object to be manufactured is prepared, which CAD model is sectioned into layers, and subsequently the building material is solidified layer-wise corresponding to cross-sections of the CAD model.
For each powder layer the building process proceeds as follows:
After the application of a building material layer in powder form onto an already solidified layer, at first the newly applied powder layer is pre-heated by means of a radiant heating up to a working temperature below the temperature at which solidification is carried out. Then, a radiation (such as a laser beam) is directed onto the pre-heated layer for selectively solidifying the building material. After all layers of the object have been manufactured, the object is allowed to cool down and is removed from the unsolidified powder that surrounds the object.
As is apparent from the just outlined manufacturing process, before parts of a powder layer that correspond to a cross-section of the object to be manufactured are solidified by sintering or melting by means of a laser beam, the powder layer at first is pre-heated. Due to the pre-heating of the powder layer the laser beam needs to supply to the powder only a small surplus energy in order to let the solidification process happen. This allows for a quicker selective solidification of a layer (selective exposure of an object cross-section that corresponds to this layer) and moreover allows for a better control of the energy introduced into the powder.
WO 95/11100 A1 deals with the manufacturing of metal parts from a metal powder and describes the problem that with a constant laser power parts having non-uniform material properties are produced. The reason for this is that there are regions of the powder bed that melt completely and regions, in which there does not occur a complete melting. Temperature inhomogeneities within a powder layer on which the laser beam for solidification is incident, are identified by WO 95/11100 A1 as cause of this problem.
WO 95/11100 A1 mentions that the fact, whether below the point of incidence of the laser beam in the current top-most layer, more precisely in the layer below this top-most layer, a solidification process already has occurred or not, plays a role. If below the point of incidence of the laser, thus in the layer below, there is still unsolidified powder material, heat dissipation by this unsolidified material is not very good and more energy is available for the melting process at the actual point of incidence of the laser in the current layer. Furthermore, it is explained that also the fact, how many already solidified regions in the current layer abut the current point of incidence, does play a role. For the case that many solidified regions are abutting the current point of incidence, heat energy will be dissipated better by the solidified regions than by unsolidified powder. The reason for this is that the solidified regions in WO 95/11100 A1 consist of a metal with good heat conductivity. Therefore, according to WO 95/11100 A1 more laser energy has to be supplied for a good solidification result (a melting of the powder as complete as possible).
In order to solve the just described problem, WO 95/11100 A1 suggests adapting the laser energy to the temperature of the powder layer that is detected by a temperature sensor close to the point of incidence of the laser for each point of incidence of the laser.
Also WO 2013/079581 A1, which deals with the manufacturing of molds from metal or ceramics powders, recognizes the necessity of taking into account temperature inhomogeneities within a layer to be solidified when energy is inputted by means of a laser. However, WO 2013/079581 A1 wants to avoid the relatively large effort in measuring technology as well as the effort for the real time computing for a temperature detection close to the respective points of incidence of the laser beam. Rather, WO 2013/079581 A1 suggests calculating by means of a computer the heat energy to be inputted at a position already before the execution of the entire building process:
As the CAD data of the object to be manufactured are at hand anyhow, with knowledge of the thermal parameters of the building material that is used the heat dissipation capability can be calculated in advance for each point of incidence of the beam in each layer and the energy input per unit time can be varied accordingly at the respective irradiation position. Here, in WO 2013/079581 A1 the different heat dissipation capability of the surrounding of a point of incidence of the beam is taken into consideration by defining a local surrounding area of a defined size around each point of incidence of the beam and calculating the heat dissipation capability for each voxel in this local surrounding area. As is already apparent from the description of the method, the computing effort for a corresponding determination of the laser parameters for each irradiation position is not small.